Clock from 1776 just goes on and on

The final masterpiece of the world's greatest clockmaker is to be put through its paces at last, 230 years after it was finished, to see if it fulfils its maker's specifications.

The priceless Late Regulator clock took John Harrison, the pioneer of longitude, 36 years to build and he was still calibrating it when he died at his home in London on March 24, 1776, his 83rd birthday.

Harrison believed that the Late Regulator would vary by only a second every 100 days and a trial has started at the Royal Observatory, Greenwich, to see if it was capable of the feat, which was first managed by an electro-mechanical timepiece in the 1920s.

The Late Regulator, also known as the RAS Regulator, because it is on loan from the Royal Astronomical Society, has been restored to its former glory to coincide with the opening at the observatory on Wednesday of a series of new galleries dedicated to time and space.

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The timepiece's blend of lignum vitae wood, brass, bronze and steel components was designed to compensate for changes in temperature and atmospheric pressure over the seasons.

Jonathan Betts, the head of horology at the National Maritime Museum, Greenwich, said: "Originally Harrison used the movement of stars past a neighbour's chimney and his window frame to measure the effects of calibration, to see if the Late Regulator ticked with the same regularity in the summer as the winter. It was a huge task and very time-consuming."

To calibrate the clock, albeit with the help of modern technology, Mr Betts has had to decipher Harrison's last known manuscript, A Description Concerning Such Mechanism as Will Afford a Nice or True Mensuration of Time.

Mr Betts said: "On first reading it sounds like gibberish but some of the concepts were new, so he had to invent names for them, such as dominion."

Harrison went against the grain of contemporary thinking by using large pendulum swings, enlarging the pendulum's "dominion" to reduce errors.

The clock has been installed on an ultra-stable concrete mounting for the observatory test.

"A lot of Harrison fans are keen to know how it will do," Mr Betts said.

Harrison used his clocks as time standards for the marine chronometers he had pioneered to deliver accuracy great enough to allow the determination of longitude at sea.

It was his fourth timekeeper, known today as H4, that finally won him the £20,000 Longitude Prize, a fortune for those days because the project was the greatest scientific, economic and political problem of the age.

The Late Regulator would have been used as the time standard for the H6 pocket timekeeper - "the lesser watch", as he called it.

Harrison's notes and drawings suggest that H6 was built but it has never been found. It looked like an overgrown pocket watch and Harrison scholars still dream of finding it in the attic.

Among Harrison's many remarkable innovations was the gridiron mechanism, consisting of alternating brass and iron rods assembled so that expansion and contraction rates cancelled each other out as the chronometer moved from the tropics to colder climes.

He was also the inventor of the first caged roller bearing, the father of the ball bearing, in his last clock.

Being almost frictionless, the Late Regulator required no oil for lubrication and therefore no cleaning.

"No other mechanical clock had ever been made to work without oil," said Mr Betts, explaining how Harrison decided to do without by using rolling bearings instead of sliding ones. "It just goes on and on and on and never has to be taken apart."

David Rooney, the curator of timekeeping, said the Late Regulator "was Harrison's last word in precision pendulum clock design. It is fantastically well designed and built - an extraordinary thing."